A number of recent findings have rekindled interest in the decades-old idea that neurons in the posterior region of the thalamus play important roles in nociception. For example, Craig et al. (94) have proposed that a nucleus that they have named VMpo is crucial in nociceptive and thermosensory processing. However, the evidence underlying Craig's proposal has recently been strongly criticized (Willis et al 2001). Indeed, Jones (2001), probably the leading authority on the thalamus, has questioned both the importance even the existence of VMpo. Thus, although substantial evidence exists for an important role for the region of the posterior thalamus in nociception, fundamental questions about the area remain unanswered. It is not clear what types of information spinothalamic tract axons carry to the posterior thalamus in rats and primates. It is also unclear which of the many nuclei within this region receive nociceptive spinal projections. In addition, the cortical regions that receive direct projections from nociceptive neurons in the posterior thalamus have not been determined. In the studies proposed in Specific Aim 1, spinal neurons that project to the posterior thalamus of rats will be identified using systematic micro-antidromic activation techniques. These studies will provide information on the specific nuclei within the posterior thalamus that receive a direct projection from the spinothalamic tract. The response characteristics of such neurons and the locations of their cell bodies within the spinal cord gray matter will also be determined. Our preliminary evidence suggests that a reasonable working hypothesis is that the triangular nucleus of the posterior thalamus is the primary recipient nucleus for nociceptive information in the region, not VMpo. In studies proposed in Specific Aim 2, somatic sensory neurons throughout the posterior thalamus of rats will be physiologically characterized and their precise cortical projections will be established. Based again on our preliminary data, our working hypothesis is that the triangular nucleus contains a large percentage of nociceptive neurons. These studies should reveal precisely the cortical areas that receive a direct nociceptive projection from the posterior thalamus. This would be strong evidence would provide new insights into the nature and locations of areas of nociceptive processing in cortex. In the studies proposed in Specific Aim 3, spinal neurons that project to the posterior thalamus will be examined in monkeys. The proposed studies will provide important information on the nature of spinal input to the posterior thalamus in primates and will reveal which area or areas in posterior thalamus of primates receive input through this large direct spinal projection.